Combustion method with selective cooling and controlled fuel mixing

ABSTRACT

New combustors, and methods of operating same, which produce lower emissions, particularly lower emissions of nitrogen oxides. Method and means are provided for reducing the flame temperature in a primary combustion zone of said combustors.

United States Patent [191 Quigg et al.

[ COMBUSTION METHOD WITH SELECTIVE COOLING AND CONTROLLED FUEL MIXING[75] Inventors: Harold T. Quigg; Robert M.

Schirmer, both of Bartlesville, Okla.

[73] Assignee: Phillips Petroleum Company,

Bartlesville, Okla.

[22] Filed: Dec. 15, 1971 [2]] Appl. No.: 208,247

[52] US. Cl 60/39.06, 60/39.5l, 60/3965,

60/3974, 431/10 [51] Int. Cl. F026 7/26 [58] Field of Search.....60/3906, 39.02, 39.51,

[56] Referenees Cited .UNITED STATES PATENTS 4/1958 Ogilvie 60/3974 R [m3 ,826,079 [4 July 30, 1974 3,067,582 12/1962 Schirmer 60/3906 3,630,02412/1971 Hopkins 60/39.69 3,684,186 8/1972 Hclmrich 60/3974 3,705,492l/l97l Vickers 60/39.5l 3,736,747 6/1973 Warren 60/DIG. 10

Primary ExaminerCarlton R. Croyle Assistant Examiner-Warren Olsen [.57]ABSTRACT New combustors, and methods of operating same, which producelower' emissions, particularly lower emissions of nitrogen oxides.Method and means are provided for reducing the flame temperature in aprimary combustion zone of said combustors.

6 Claims, 1] Drawing Figures PATENTEB M30!!!" SHEET 10F 4 ATTORNEYSPAIENIEB JUL 3 0 I974 SHEET 3 BF 4 INVENTORS H. T. QU 66 R M. SCHIRMERATTORNEYS mcmwmm 3.826.079

FUEL

INVENTOR. H.T. QUIGG RMSCHIRMER A TTORNEVS COMBUSTION METHOD WITHSELECTIVE COOLING AND CONTROLLED FUEL MIXING V This invention relates toimproved gas turbine combustors and methods of operating same.

Air pollution has become a major problem in the United States and otherhighly industrialized countries of the world. Consequently, the controland/or reduction of said pollution has become the object of majorresearch and development effort by both governmental and nongovernmentalagencies. Combustion of fossil fuel is a primary source of saidpollution. It has been alleged, and there is supporting evidence, thatautomobiles employing conventional piston-type engines are a majorcontributor to said pollution. Vehicle emission standards have been setby the United States Environmental Protection Agency which aresufficiently restrictive to cause automobile manufacturers to consideremploying alternate engines instead of the conventional piston engine.

The gas turbine engine is being given serious consideration as analternate engine. However, insofar as we presently know, there is nopublished information disclosing realistic and/or practical combustorswhich can be operated at conditions typical of those existing in highperformance engines, and which will have emission levels meeting orreasonably approaching the standards set by said United StatesEnvironmental Protection Agency. This isparticularly true with respectto nitrogen oxides emissions.

Thus, there is a need for a combustor of practical and/or realisticdesign which can be operated in a manner such that the emissionstherefrom will meet said standards. Even a practical combustor givingreduced emissions (compared to the combustors of the prior art)approaching said standards would be a great ad vance in the art. Such acombustor would have great potential value because it is possible thepresently very restrictive standards may be relaxed.

The present invention solves the above-described problems by providingimproved combustors, and methods of operating same, which produceemissions meeting or reasonably approaching the present stringentstandards established by said Environmental Protection Agency. Saidmethods comprise a combination of (a) controlled mixing of fuel and airwithin the flame tubes of said combustors, and (b) reduction of theflame temperature within said flame tubes Thus, according to theinvention, there is provided a combustor, comprising, in combinationan'outer casing; a flame tube disposed concentrically within said casingand spaced apart therefrom to form an annular chamber between said flametube and said casing; a plurality of fins extending from the externalsurface of said flame tube into said annular chamber; air inlet meansfor introducing a swirling stream of air into the upstream end portionsof said flame tube; and fuel inlet means for introducing a stream offuel into said flame tube in a direction which is from tangent to lessthan perpendicular, but non-parallel, to the periphery of said stream ofair.

Further according to the invention, there is provided a method forreducing the formation of nitrogen oxides formed in the combustion of afuel in a combustor,

which method comprises: introducing a swirling stream of air into theupstream end portion of a combustion zone; forming and introducing anannular stratum of said fuel around said stream of air and in adirection which is from tangent to less than perpendicular, butnon-parallel, to the periphery of said stream of airso as to effectcontrolled mixing of said fuel and air at the interface therebetween;burning said fuel; and decreasing the flame temperature within saidcombustion zone by removing heat from an extended outer wall surface ofsaid combustion zone.

FIG. 1 is a view in cross section of a combustor in accordance with theinvention.

FIG. 1-A is a schematic representation of fuel and air introduction inaccordance with the invention.

FIG. 2 is a cross section taken along the line 2-2 of FIG. 1.

FIG. 3 is a view in cross section of a portion of the flame tube and aclosure member therefor of another combustor in accordance with theinvention. The outer housing or casing and other elements of thiscombustor is substantially like that shown in FIG. 1.

FIG. 4 is a view in cross section taken along the line 4-4 of FIG. 3.

FIG. 5 is a view in cross section taken along the line 5-5 of FIG. 1.

FIG. 6 is a view in cross section taken along the line 6--6 of FIG. 1.

' FIG. 7 is a perspective view of a portion of a flame tube showing analternate type of. fin which can be employed. V I

FIG. 8 is a view in cross section taken along the line 8-8 of FIG. 3.

FIGS. 9 and 10 are views in cross section of other closure members ordome members which can be employed with the flame tubes of the othercombustors described herein.

' Referring now to the drawings wherein like reference numerals areemployed to denote like elements, the invention will be more fullyexplained. v

In FIG. 1 there is illustrated a combustor in accordance with theinvention, denoted generally by the referencenumeral 10, which comprisesa flame tube 12. Said flame tube 12 is open at its downstream end, asshown, for communication with a conduit leading to a turbine or otherutilization of the combustion gases. A closure member, designatedgenerally by the-reference numeral 14, is provided for closing theupstream end of said flame tube. Said closure member can be. fabricatedintegrally, i.e., as one element, if desired. However, it is presentlypreferred to fabricate said closure member 14 as two or more elements,e.g.,an upstream element 16 and a downstream element 18. An outer casing20 is disposed concentrically around said flame tube 12 and saidclosuremember 14 and is spaced apart therefrom to form an annularchamber 22 around said flame tube and said closure member. Said annularchamber 22 is closed at its downstream end by any suitable means such asthat illustrated. A plurality of fins 23, or other type of turbulators,extend from the outer surface of said flame tube 12 into said annularspace 22. Preferably, said fins are provided on said flame tube in theregion between the downstream end portion of said closure member and thefirst set of openings 38. However, it is within the scope of theinvention to provide fins essentially the full length of flame tube 12,e.g., to openings 39. As shown in FIGS. 1, 5, and 6, said fins arearranged in alternate spaced apart rows around said flame tube and arespaced apart from each other in each row. Said fins can extend fromflame tube 12 into annular space 22 any convenient and suitabledistance,

but preferably extend the full distance to outer casing 20, as shown.FIG. 7 illustrates an alternate type of longitudinal fln 23' which canbe employed if less pressure drop in annular space 22' is desired. Saidfins 23 and 23' thus form an extended surface on the outer wall of flametube 12. Any other suitable type of fin or extended surface can beemployed.

Suitable flange members, as illustrated, are provided at the downstreamend of said flame tube 12 and outer housing for mounting same andconnecting same to a conduit leading to a turbine or other utilizationof the combustion gases from the combustor. Similarly, suitable flangemembers are provided at the upstream end of said flame tube 12 and saidouter housing 20 for mounting same and connecting same to a conduit 24which leads from a compressor or other source of air. While not shown inthe drawing, it will be understood that suitable support members areemployed for supporting said flame tube 12 and said closure member 14 inthe outer housing 20 and said upstream end flange members. Saidsupportmembers have been omitted so I as to simplify the drawing.

A generally cylindrical swirl chamber 26 is formed in said upstreamelement 16 of closuremember 14. The downstream end of said swirl chamber26 is in-open communication with the upstream end of said flame tube 12.A first air inlet means is provided for introducing a swirling mass ofair into the upstream end portion of said swirl chamber 26 and then intothe upstream end of said flame tube. As illustrated in FIGS. 1 and 2,said air inlet means comprises a plurality of air conduits 28 extendinginto said swirl chamber 26 tangentially with respect to the inner wallthereof. Said conduits 28 extend from said annular passageway or chamber22 into said swirl chamber 26.

A fuel inlet means is provided for introducing a stream of fuel in adirection which is from tangent to less than perpendicular, butnon-parallel, to the periphery of said stream of air. As illustrated inFIGS. 1 and 2, said fuel inlet means comprises a fuel conduit 30 leadingfrom a source of fuel, communicating with, a passageway 32,. which inturn communicates with fuel passageway 34 which is formed by an innerwall of said downstream element 18 of closure member 14 and thedownstream end wall of said upstream. element 16 of closure member 14.It will be noted that the inner wall of said downstream element isspaced apart from and is complementary in shape to the downstreamendwall of said upstream element 16. The direction of the exit portion ofsaid fuel passageway 34 can be varied over a range which is intermediateor between tangent and perpendicular, but non-parallel, to the peripheryof the stream of air exiting from swirlchamber 26. Varying the directionof the exit portion of fuel passageway 34 provides one means or methodfor controlling the degree of mixing between the fuel stream and saidair stream at the interface therebetween. As illustrated in FIG. 1, thedirection of the exit portionof fuel passageway 34 forms an angle ofapproximately 45 with respect to the periphery of the air exiting .fromswirl chamber 26. Generally speaking, in most instances, it will bedesired that the exit portion of said fuel passageway 34 has a directionwhich forms an angle within the range of from about 15 to about 75,preferably about 30 to about 60 with respectto the periphery of thestream of air exiting from swirl chamber 26. In most instances, it willbe preferred that the fuel from fuel passageway 34 be introduced in agenerally downstream direction. However, it is within the scope of theinvention to introduce said fuel in an upstream direction. Shim 36provides means for varying the width of said fuel passageway 34. Anyother suitable means, such as to each other, so as to accommodate theabovedescribed changes indirection and width of said fuel passageway 34.i

A plurality of openings 38 is provided at a first station located in thedownstream portion of said flame tube 12 for admitting a second streamof air into said flame tube from said annular chamber 22. Preferably, asec: ond plurality of openings 39 is provided at a second stationdownstream and spaced apart, from said openings 38 for admitting a'thirdstream of air into said flame tube from said annular chamber 22. Whenonly one set of openings such as 38 is provided, said second stream ofair will comprise principally quench air for quenching the combustionproducts before passing same on to the turbine. When two sets ofopenings such as 38 and 39 are provided, said second stream of air will,comprise principally secondary, air, and said third stream of air willcomprise principally quench air. Varying the distance between saidopenings 38 and 39 provides a method of controlling carbon monoxideemissions. Generally speaking, increasing said distance will decreasecarbon monoxide emissions.

Referring now to FIG. 3, there is illustrated a portion of the flametube and a closure member therefor of another combustor in accordancewith the invention. It will be understood that the complete combustorwill comprise an outer housing creasing 20 and suitable flange memberssubstantially like that illustrated in FlG.,1.' The flame tube 12 of thecombustor of FIG. 3 is like flame tube 12 of FIG. 1. A closure member 40is mounted onthe upstream end of said flame tube 12 in any suitablemanner so as to close the upstream end of said flame tube except for theopenings provided in said closure member. A generally cylindrical swirlchamber 42 is formed in said closure member 40. The downstream end ofsaid swirl chamber is in open communication with theupstream end of saidflame tube. An air inlet means is provided for introducing a swirlingmass of airinto the upstream'end portion of said swirl chamber 42 andthen into the upstream end of said flame tube 12. As illustrated inFIGS. 3 and 4, said air inlet means comprises a'plurality of airconduits 44 extending into said swirl chamber 42 tangentially withrespect to the inner wall thereof. Said conduits 44 extend from anannular chamber 22, similarly as in FIG. 1. The fuel inlet means in thecombustor of FIG. 3 comprises a fuel supply conduit .46 which is incommunication with three fuel passageways 48, which in turn is incommunication with an annular fuel passageway 51 formed in thedownstream end portion of said closure member 40. A plurality of fuelconduits 49 extend from said passageway 51 into a recess 50 formed inthe downstream end portion of said closure member, and tangentially withrespect to the inner wall of said recess. As illustrated in FIGS. 3 and4, said air inlet conduits 44' are adapted to introduce air tangentiallyinto swirl chamber 42in a clockwise direction (when looking downstream),and said fuel inlet conduits 49 in'.FlG. 8 are adapted to introduce fueltangentially into said recess 50 in a counterclockwise direction. Thisis a presently preferred arrangement in one embodiment of the invention.However, it is within the scope of the invention to reverse thedirections of said air inlet conduits 44 and said fuel inlet conduits49, or to have the directions of both said air inlet conduits and saidfuel inlet conduits the same, e.g., both clockwise or bothcounterclockwise.

Referring now to FIGS. 9 and 10, there are illustrated other types ofclosure members which can be employed with the flame tubes of thecombustors described above. In FIG. 9 closure member 78 is similar toclosure member 40 of FIG. 3. The principal difference is that in closuremember 78 a conduit means 80 is provided which extends through saidclosure member 78 into communication with the upstream end portion offlame tube.12, for example. At least one swirl vane 82 is positioned insaid conduit means 80 for imparting a swirling motion to the air passingthrough said conduit means 80. In FIG. 10, closure member 84 is similarto closure member 14 of FIG. 1. The principal difference is that inclosure member 84 an annular conduit means 88 is provided which extendsthrough the body of said closure member 84 into open communication withthe upstream end of the flame tube 12, for example.- At least one swirlvane 90 is provided in said conduit means 88 for imparting a swirlingmotion to the air passing through said conduit 88.

In one presently preferred method of operating the combustor of FIG. 1,a stream of air from a compressor (not shown) is passed via conduit 24into annular space 22. A portion of said air then passesthroughtangential conduits 28 into swirl chamber 26. Said tangentialconduits 28 impart a helical or swirling motion to the air entering saidswirl chamber and exiting therefrom. This swirling motion creates avortex action resulting in a reverse circulation of hot gases withinflame tube 12 upstream toward said swirl chamber 26 during operation ofthe combustor.

A stream of fuel, preferably prevaporized, is admitted via conduit 30,passageway 32, and fuel passageway 34. Fuel exiting from fuel passageway34 is formed into an annular stratum around the swirling stream of airexiting from swirl chamber 26. This method of introducing fuel and aireffects a controlled mixing of said fuel and air at the interfacetherebetween. Initial contact of said fuel and air occurs upon the exitof said air from said swirl chamber 26. Immediately after said initialcontact the fuel and air streams (partially mixed at said interface) areexpanded, in a uniform and graduated manner during passage of said fueland air through the flared portion of member 18, from the volume thereofin the region of said initial contact to the volume of said combustionchamber at a point in said flame tube downstream from said initialcontact. Said expansion of fuel and air thus takes place during at leasta portion of the mixing of said fuel and said air. The resultingr'nixture of fuel and air is burned and combustion gases exit tube. Athird stream of air, comprising quench air, is admitted tothe interiorof flame tube 12 from annular space 22 via inlet openings39. Said secondand third streams of air in passing over fins 23 in annular space 22extract heat from said tins (and the interior of flame tube 12) and thusreduce the flame temperature in said flame tube.

In one presently preferred method of operating the combustor of FIG. 3,the method of operation is similar to that described above for thecombustor of FIG. 1. A stream of air is admitted to swirl chamber 42 viatangential inlet conduits 44 which impart a helical or swirling motionto said air; A stream of fuel, preferably prevaporized, is admitted viaconduit 46, fuel passageways 48, and tangential fuel conduits 49 intorecess 50 formed at the downstream end of said closure member 40. Saidfuel is thus formed into an annular stratum EXAMPLES As series of testruns was made employing a combustor of the invention described herein,and'a typical standard or prior art combustor as a control combustor.The same fuel wasused in all of said test runs. Properties of said fuelare set forth in Table 1 below.

' Design details of the combustor of the invention are set forth inTable II below. Said design details, e.g dimensions, are given by way ofillustration only and are not to be construed as limiting on theinvention. Said dimensions can be varied within wide limits so long asthe improved results of the invention are obtained. For example, theformation of nitrogen oxides in a combustion zone is an equilibriumreaction. Thus, in designing a combustion zone, attention should begiven to the size thereof so as to avoid unduly increasing the residencetime therein. It is desirable that said residence time not be longenough to permit the reactions involved in the formation of nitrogenoxides to attain equilibrium.

Said control combustor basically embodies the principal features ofcombustors employed in modern aircraft-turbine engines. It is astraight-through can-type combustor employing fuel atomization by asingle simplex-type nozzle. The combustor liner was fabricated from2-inch pipe, with added internal deflector skirts for air film coolingof surfaces exposed to the flame. Exhaust emissions from this combustor,when operated at comparable conditions for combustion, are in generalagreement with measurements presently available from several differentgas turbine engines. Said control combustor had dimensions generallycomparable to the above described combustors of the invention.

The combustor of the invention and said control combustor were run at 12test points or conditions, i.e.,

12 different combinations of inlet-air temperature, combustor pressure,flow velocity, and heat input rate. Test points or conditions I to 6simulate idling conditions, and test points 7 to 12 simulate maximumpower conditions. The combustor of the invention was run usingprevaporized fuel. The control combustor was run using atomized liquidfuel. In all runs the air stream to the combustors was preheated byconventional means. Analyses for content of nitrogen oxides (reported asNO), carbon monoxide, and hydrocarbons (reported as carbon) in thecombustor exhaust gases were made at each test condition for eachcombustor. The method for measuring nitrogen oxides was based on theSaltzman technique, Analytical Chemistry 26,

TABLE II TABLE I PHYSICAL AND CHEMICAL PROPERTIES OF 1 5 COMBUSTORDESIGN TEST FUEL Philjet A -50 Variable Invention Combustor ASTMDistillation, F. Closure Member (14) lmtlal B iling P int 340 20 Airlnlet Diameter, in. 0.875 5 vol evaporated 359 lnlet Type Tangent volevaporated 2 Hole Diameter, in. 0.188 vol evaporated 371 Number f Hol s6 vol evaporated 376 Total Hole Area, sq. in. 0.166 40 vol evaporated387 Total Combustor Hole Area 3.213 vol evaporated 398 Fuel Slot, in.0.005 vol evaporated 409 25 vol evaporated 424 vol evaporated 442 Flamez fi g z (38) vol vevaporaed Hole Diameter, in. 5/ 16X] vol evaporated474 Total Number of Holes 8 g g l Total Hole Area, sq. in. 2.500 Lestueivz b Total Combustor Hole Area 48.393

9 30 2nd Station 39) i dean AH Hole Diameter. in. 5/l6l Total Number ofHoles 8 Heat .of Combustion. net, Btu/lb. 18,670 Tom Hole Area sq. in2500 t b Total Combustor Hole Area 48.393 g g e & mm 0 d Combustor CrossSect. Area, sq. in. 3.355 u wt Total Combustor Hole Area, sq. in. 5.166il ml Cross Sectional Area 153.933 Compositlon, vol 35 P ff'ns 52.8 w 345 Combustor Diameter, in. 2.067 O| fi Primary Zone Length, in. 3.375Aromatics Volume, cu in. 11.326 7 Combustor Length, in. 1 1.250 Formula(calculated) (c n 40 volume, m 1

Stoichiometric Fuel/Air Ratio, lb/lb TABLE III.--COMPARISON OE EMISSIONSFROM COMBUSTORS AT IDLE CONDITIONS Holes are 5/ l 6" diameter at ends;slots are I" long. Cooling fins were 4 7.??? r Test conditions,lbs/1,000 lbs. fuel 7 Combustor operating variables:

Temperature, inlet air. F 900 900 900 900 900 Pressure, in. abs... S0 5050 50 50 50 Velocity, col ow, ft sec. 250 250 250 400 400 400treat-input rate, Btu/lb. air, 200 275 350 200 275 350 NITR XIDES:

Combustors:

Control combustor 3.4 3 4 3.2 2.2 2.1 2.3 Invention combustor 1.8 16 2.62.0 2.2 1.9 CARBON MONOXIDE;

Combustors:

Control Combustor .I 10 2 0 l7 9 0 Invention combustor 5 4 0 5 l4 3HYDROCARBONS:

Combustors:

Control combustor 0.6 0.7 0.4 0.9 0.4 0.8 Invention combustor 0.4 0.40.2 0.4 2.2 0.6

Referring to the above Table III, the data there given clearly show thatthe combustor of the invention gave results superior to theresults'obtained with the control combustor. The invention combustorgave superior results at substantially all test conditions with respectto nitrogen oxides emissions, the pollutant most difficult to control.Said data also show that combustors in accordance with the invention canbe operated at idle conditions to give not more than about 2.5,preferably not more than 2 pounds of nitrogen oxides emissions per 1,000pounds of fuel burned, and not more than about 5 pounds of nitrogenoxides emissions per 1,000 pounds of fuel burned at maximum powerconditions.

' vention was prevaporized. However, the invention is not limited tousing prevaporized fuels and it is within the scope of the invention toemploy atomized liquid fuels. For comparison purposes, all the runs setforth in the above examples were carried out under the condi torpressures within the range of from about 1 to aboutatmospheres orhigher; at flow velocities within the range of from about 1 to about 500ft. per second or higher; and at heat input rates within the range offrom about 30 to about 1,200 Btu per pound of air. Generally speaking,operating conditions in the combustors of the invention will depend uponwhere the combustor is employed. For example, when the combustor isemployed with a high pressure turbine, higher pressures and higher inletair temperatures will be employed in;

the combustor. Thus, the invention is not limited to any particularoperating conditions.

'The term air" is employed generically herein and in the claims, forconvenience, to include air and other combustion supporting gases.

While the invention has been described, in some in stances, withparticular reference to combustors employed in combination with gasturbine engines, the in- 1 9 10 T glCOMPARISON OF EMISSIONS FROMCOMBUSTORS AT MAXIMUM POWER I CONDITIONS Testconditions, lbs.'/l,000lbs. fuel Combustor operating variables:

Combustors: Control combustor........................... 10.7 11.2 10.09.9 8.0 7.4? Invention combustor........................ 5.1 4.2 7.1 6.74 .2 5.0i CARBON MONOXIDE: 5

Combustors: 1

Control combustor 0 0 0 0 0 01 Inventioncombustor......................... 0 0 0 0 0 0e HYDROCARBONS: vcombustors: i .9 wJP b ..9 0; :2 I Invention combustor 1.0 0.3 0.2 0.40.2 0.1 -T

vention is not limited thereto. The combustors of the invention haveutility in other applications,'e.g., boilers and other stationarypower-plants. 1

Thus, while certain embodiments of the inventio have been described forillustrative-purposes, the invention is notlimited thereto. Variouso'ther modifications or embodiments of the invention will be apparent tothose skilled in the art in view of this disclosure. Such modificationsor embodiments are within the spirit and scope of the disclosure.

l. A method for reducing the formation of nitrogen oxides formed in thecombustion of a fuel in a combustor, which method comprises:

introducing a swirling stream of air into the upstream end portion of acombustion zone as the sole stream of primary air introduced into saidcombustion zone;

forming and introducing an annular stratum of said fuel around saidstream of air by introducing said fuel in a direction toward and whichis from tangent to less than perpendicular, but non-parallel, to theouter periphery of said stream of air so as to effect controlled mixingof said fuel and air at the interface to produce an annular fuel-airmixture;

passing said fuel-air mixture into said combustion zone as the solefueland air supplied to. the upstream portion of said combustion zone;

burning said fuel; and

decreasing the flame temperature within said combustion zone by removingheat from an extended outer wall surface of said combustion. zone.

2. A method according to claim 1 wherein said fuel and said air areexpanded during at least a portion of said mixing thereof, and saidexpansion of said fuel and said air is initiated immediately after theinitial contact at said interface therebetween'.

3. A method according to claim 2 wherein: said stream of air isinitially introduced into a swirl zone having a diameter less than thediameter of thereof in the region of said initial contact to the l l 12volume of said combustion chamber at a point in nd stream of air oversaid extended outer wall sursaid combustion chamber downstream from saidface of said combustion zone; and initialcontact. I said second streamof air is introduced into said com- 4. A method according to claim 3wherein said fuel 5 g Zone -dowl?st-ream from thepomt of muouction ofsaid swirling stream of air. Ptroduced .angle wnhm the rangeoffr9m'about 6. A method according to claim 5 wherein a third to about75 .wlth respect to the outer periphery of stream of air is introducedinto said combustion zone said stream of downstream from the point ofintroduction of said sec- 5. A method according to claim 1 wherein: 0ndstream of air.

said flame temperature is reduced by passing a sec- 10

1. A method for reducing the formation of nitrogen oxides formed in the combustion of a fuel in a combustor, which method comprises: introducing a swirling stream of air into the upstream end portion of a combustion zone as the sole stream of primary air introduced into said combustion zone; forming and introducing an annular stratum of said fuel around said stream of air by introducing said fuel in a direction toward and which is from tangent to less than perpendicular, but non-parallel, to the outer periphery oF said stream of air so as to effect controlled mixing of said fuel and air at the interface to produce an annular fuel-air mixture; passing said fuel-air mixture into said combustion zone as the sole fuel and air supplied to the upstream portion of said combustion zone; burning said fuel; and decreasing the flame temperature within said combustion zone by removing heat from an extended outer wall surface of said combustion zone.
 2. A method according to claim 1 wherein said fuel and said air are expanded during at least a portion of said mixing thereof, and said expansion of said fuel and said air is initiated immediately after the initial contact at said interface therebetween.
 3. A method according to claim 2 wherein: said stream of air is initially introduced into a swirl zone having a diameter less than the diameter of said combustion zone; said initial contact between said fuel and said air occurs upon the exit of said air from said swirl zone; and said expansion of said fuel and said air occurs in a uniform and graduated manner from the volume thereof in the region of said initial contact to the volume of said combustion chamber at a point in said combustion chamber downstream from said initial contact.
 4. A method according to claim 3 wherein said fuel is introduced at an angle within the range of from about 15* to about 75* with respect to the outer periphery of said stream of air.
 5. A method according to claim 1 wherein: said flame temperature is reduced by passing a second stream of air over said extended outer wall surface of said combustion zone; and said second stream of air is introduced into said combustion zone downstream from the point of introduction of said swirling stream of air.
 6. A method according to claim 5 wherein a third stream of air is introduced into said combustion zone downstream from the point of introduction of said second stream of air. 